.travis.yml in unsplash-1.5.6 vs .travis.yml in unsplash-2.0.0

@@ -1,6 +1,7 @@
 language: ruby
 rvm:
-  - 2.2.5
-  - 2.3.1
-  - 2.4.0
-before_install: gem install bundler -v 1.10.2
+  - 2.3.8
+  - 2.4.5
+  - 2.5.4
+  - 2.6.2
+before_install: gem install bundler